A 7500-year strontium isotope record from the northwestern Nile delta (Maryut lagoon, Egypt)

During the Holocene, delta evolution has been collectively mediated by relative sea-level changes, continental hydrology and human impacts. In this paper, we present a strontium isotope record from the Maryut lagoon (northwestern Nile delta) to quantify the interplay between relative sea-level variations and Nile flow changes during the past 7500 years. ⁸⁷Sr/⁸⁶Sr stratigraphy allows five hydrological stages to be defined. (1) The marine transgression of the area is dated to ~7.5kacal.BP, with a clear marine ⁸⁷Sr/⁸⁶Sr signature (0.70905-0.7091). (2) Between ~7 and ~5.5ka, in the context of the so-called African Humid Period (AHP), freshwater inputs became progressively predominant in the Maryut's hydrology. Deceleration of sea-level rise coupled with high Nile discharge induced coastal progradation which led to the progressive closure of the Maryut lagoon. (3) Between ~5.5 and ~3.8ka, the end of the AHP is translated by a progressive hydrological shift from a Nile-dominated to a marine-dominated lagoon (⁸⁷Sr/⁸⁶Sr shifts from 0.70865 to 0.7088 to 0.70905-0.70915). (4) From ~2.8 to ~1.7ka, ⁸⁷Sr/⁸⁶Sr ratios shift towards lower values (0.7084). Although this change is not precisely resolved because of a hiatus in the Maryut's sedimentary record, the ⁸⁷Sr/⁸⁶Sr transition from sea-like to Nile-dominated values is attributed to irrigation practices since the early Ptolemaic period (i.e. since ~2.3ka), including the Alexandria canal which played a key role in isolating the Maryut from the Mediterranean sea. (5) The final phase of the record covers the period between ~1.7 and ~0.2ka. ⁸⁷Sr/⁸⁶Sr ratios indicate high freshwater inputs (from 0.7080 to 0.7085), except between 1.2 and 1.1 to ~0.7ka, when a Maryut lowstand and seawater intrusion are attested. In modern times, the Nile's coastal lagoons have been increasingly supplied by freshwater linked to the diversion of waters from the two Nile branches into the irrigation system. It is suggested that this process began in early Antiquity and has engendered a reduction in the number of Nile branches from seven in ancient times to just two at present.